Hitherto, piezoelectric resonators have been used for various piezoelectric resonance parts such as piezoelectric oscillators and piezoelectric filters. Furthermore, piezoelectric resonators using various piezoelectric vibration modes according to a used frequency are known.
For example, in Patent Document 1 described below, a thickness extensional piezoelectric resonator 101 shown in FIG. 22 is disclosed. The thickness extensional piezoelectric resonator 101 is an energy-trapping-type piezoelectric resonator that uses a third-order harmonic of a thickness extensional vibration mode. Here, a strip piezoelectric body 102 in the shape of a rectangular plate is used. On the top surface of the piezoelectric body 102, a first resonance electrode 103 is formed, and on the bottom surface, a second resonance electrode 104 is formed. The bottom surface of and the top surface of the resonance electrodes 103 and 104, respectively, oppose each other through the piezoelectric body 102 in the central portion of the piezoelectric body 102 along the length direction thereof. The resonance electrodes 103 and 104 are formed so as to span the full width of the piezoelectric body 102.
When the width of the piezoelectric body 102 is denoted as W, the thickness is denoted as t, and d=t/3, W/d is set to 7.7 or less. As a result, an unwanted spurious frequency that appears when a third-order harmonic of thickness extensional vibration is used can be suppressed.
On the other hand, in Patent Document 2 described below, a similar energy-trapping-type piezoelectric resonator using a third-order harmonic of thickness extensional vibration is disclosed. Here, by setting the ratio W/T of the width W of the piezoelectric body to the thickness T to a specific range, an unwanted spurious frequency between the resonance frequency and the anti-resonance frequency and in the vicinity thereof can be effectively suppressed.
In Patent Document 3 described below, it is disclosed that, in an energy-trapping-type piezoelectric resonator using a third-order harmonic of thickness extensional vibration, for the piezoelectric body, perovskite complex oxide of a specific composition containing Pb, Ti, Li, Sr, and Mn is used, thereby improving the temperature characteristics of the oscillation frequency. That is, in Patent Document 3, it is disclosed that changes in the oscillation frequency at −20° C. to +80° C. due to temperature can be reduced.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 1999-8527
Patent Document 2: Japanese Unexamined Patent Application Publication No. 1998-290139
Patent Document 3: Japanese Unexamined Patent Application Publication No. 1999-130526
In recent years, in a piezoelectric oscillator, there has been an increasing demand for a reduction of changes in an oscillation frequency due to temperature. For example, in the range of temperature of 0 to 70° C., there has been a demand for changes in the oscillation frequency to be in the range of approximately ±100 ppm at room temperature.
In the piezoelectric oscillator using a third-order harmonic of a thickness extensional vibration mode, which is disclosed in Patent Document 3 described above, changes in the oscillation frequency due to temperature are reduced by using a piezoelectric body of a specific composition. However, with only such a selection of materials, it has been very difficult to make the amount of changes in the oscillation frequency to be within ±100 ppm, for example, in the temperature range of 0 to 70° C. For example, in a piezoelectric oscillator indicated by No. 14 in FIG. 1 in Patent Document 3, the above-described demand is not satisfied.
Furthermore, the frequency-temperature characteristics of the piezoelectric resonator change according to various conditions, such as a polarization voltage when piezoelectric bodies to be used are manufactured or a heat treatment temperature after polarization. Therefore, even if temperature characteristics are to be controlled by adjusting the composition, there is a risk that temperature characteristics become different from designed temperature characteristics depending on various conditions of actual manufacturing steps. That is, it has been very difficult to accurately set temperature characteristics only by adjusting the composition of materials from which a piezoelectric body is formed.
In addition, many experiments and much time have been necessary for developing materials, such as selection of the composition range, and it has not been easy to reliably find temperature characteristics to be required, thereby necessitating much effort and labor.
On the other hand, in piezoelectric resonators of Patent Documents 1 and 2 described above, an unwanted spurious frequency between the resonance frequency and the anti-resonance frequency is suppressed. In order to suppress such a spurious frequency, the dimensions of the piezoelectric resonator described above are set to be within a specific range. However, in Patent Documents 2 and 3, it is described that frequency characteristics are improved by suppressing or shifting the unwanted spurious frequency, but no mention is made of the temperature characteristics themselves of the resonance frequency. That is, in Patent Documents 2 and 3, suppression of spurious frequency is considered by adjusting the dimensions of a piezoelectric body. As a consequence, the piezoelectric resonator has various temperature characteristics depending on dimensions with which such suppression of the spurious frequency is possible. Therefore, it has been difficult to reliably reduce changes in characteristics due to changes with temperature. That is, it has been difficult to make the changes in characteristics ±100 ppm or less in the range of temperature of 0 to 70° C.